The `ESS_moderator_long` Component

A parametrised pulsed source for modelling ESS long pulses.

Identification

Site:
Author: KL, February 2001
Origin: Risoe
Date:

Description

Produces a time-of-flight spectrum, from the ESS parameters
Chooses evenly in lambda, evenly/exponentially decaying in time
Adapted from Moderator by: KN, M.Hagen, August 1998

2012-updates:

Geometry is now MCNPX-like IMPORTANT: Origin of the component is inside the cylindrical
moderator, i.e. take care when positioning the next components! (E Klinkby)
Component implements both the cold moderator and the thermal (pre-)moderator, fraction of statistics for
the cold moderator is the new cold_frac parameter. New set of input parameters with subscript _t defines the
thermal flux.(E Klinkby)
IMPORTANT: The thermal flux corresponds to the 2001 thermal ESS moderator as no update has currently
been released from the ESS neutronics group.
By default the component applies a wavelength-dependent correction term to the cold flux, derived from
2012 MCNPX calculations by ESS neutronics group. Corrections calculated by K Lieutenant (Vitess) and
implemented here by E Klinkby. In case this is not wanted, the src_2012 parameter can be set to 0.
Default cold moderator intensity parameters correspond to the "ESS 2012" parameter set. The original
2001 ESS "Mezei moderator" can be described by setting T=50, tau=287e-6, tau1=0, tau2=20e-6, chi2=0.9, I0=6.9e11,
I2=27.6e10, branch1=0, branch2=0.5, src_2012=0
The component can use target_index for focusing to a given beam port. Use an Arm() and ROTATED to position
relatively to the moderator.
Time focusing option: Adjusts neutron departure time to match a 'first chopper' defined by parameters tfocus_dist, tfocus_time, tfocus_width (K Lefmann).


Units of flux: n/cm^2/s/AA/ster
(McStas units are in general neutrons/second)

Example general parameters (general):
size=0.12 Lmin=0.1 Lmax=10 dist=1.6 focus_xw=0.19 focus_yh=0.15 nu=16.67

Example moderator specific parameters
(From F. Mezei, "ESS reference moderator characteristics for ...", 4/12/00:
Defining the normalised Maxwellian
M(lam,T) = 2 a^2 lam^-5 exp(-a/lam^2); a=949/T; lam in AA; T in K,
the "pulse integral" function
iexp(t,tau,d) = 0                              ; t<0
tau (1-exp(-t/tau))            ; 0d ,
and the long pulse shape function
I(t,tau,n,d) = (iexp(t,tau,d)-iexp(t,tau/n,d)) n/(n-1)/tau/d ,

the flux distribution is given as
Phi(t,lam) =  I0 M(lam,T) F(t,tau,n)
+ I2/(1+exp(chi2 lam-2.2))/lam*F(t,tau2*lam,n2)  )

c1: Ambient H20, long pulse, coupled ESS 2001 thermal
T_t=325 tau_t=80e-6 tau1_t=400e-6 tau2_t=12e-6 n=20 n2=5 d=2e-3 chi2_t=2.5
I0_t=13.5e11 I2_t=27.6e10    branch1_t=0.5 branch2_t=0.5

c2: Liquid H2, long pulse, coupled ESS 2012 cold
T=50 tau=287e-6 tau1=0 tau2=20e-6 n=20 n2=5 d=2e-3 chi2=0.9
I0=8.21e11, I2=3.29e11    branch1=0 branch2=0.5

Debugged intensively against Mezei note (4/12 2000) and VitESS @ Rencurel 2006.
The output is now neutrons / second, not as previously neutrons / pulse.

%VALIDATION
Validated against VitESS and Mezei note (4/12 2000) @ Rencurel 2006

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description	Default
width_c			0
yheight			0.12
Lmin	AA	Lower edge of wavelength distribution
Lmax	AA	Upper edge of wavelength distribution
dist	m	Distance from source to focusing rectangle; at (0,0,dist)	0
focus_xw	m	Width of focusing rectangle
focus_yh	m	Height of focusing rectangle
nu	Hz	Frequency of pulses	14
T	K	Temperature of cold source	50
tau	s	long time decay constant for cold pulse tail 1a	287e-6
tau1	s	long time decay constant for cold pulse tail 1b	0
tau2	s	long time decay constant for cold pulse, 2	20e-6
d	s	pulse length	2.857e-3
n	1	pulse shape parameter, 1	20
cold_frac	1	Fraction of neutron statistics from cold source. It is implicitely assumed that supermirror allows each beamline to choose the desired fraction of cold and thermal neutrons (i.e. extreme idealization).	1.0
n2	1	pulse shape parameter, 2	5
chi2	1/AA	lambda-distribution parameter in cold pulse 2	0.9
I0	flux	integrated cold flux, 1 (in flux units, see above)	8.21e11
I2	flux	Cold flux, 2 (in flux units, see above)	3.29e11
target_index	1	relative index of component to focus at, e.g. next is +1 this is used to compute 'dist' automatically.	0
cyl_radius	m	Radius of the cylindershaped cold source	0.085
branch1	1	limit for switching between two time structures in cold distribution 1 (only for coupled water, else = 1)	1
branch2	1	limit for switching between cold distribution 1 and 2. (default value 0.5)	0.5
branch_tail	1	limit for switching between pulse and tail (suggested value: tau/d - default defined this way)	0.14350
n_pulses	1	Number of pulses simulated. 0 and 1 creates one pulse. The integrated intensity is constant	1
width_t	m	Edge of cube shaped thermal source	0.12
T_t	K	Temperature of thermal source	325
tau_t	s	long time decay constant for thermal pulse tail 1a	80e-6
tau1_t	s	long time decay constant for thermal pulse tail 1b	400e-6
tau2_t	s	long time decay constant for thermal pulse, 2	12e-6
chi2_t	1/AA	lambda-distribution parameter in thermal pulse 2	2.5
I0_t	flux	integrated thermal flux, 1 (in flux units, see above)	13.5e11
I2_t	flux	Thermal flux, 2 (in flux units, see above)	27.6e10
branch1_t	1	limit for switching between two time structures in thermal distribution 1 (only for coupled water, else = 1)	0.5
branch2_t	1	limit for switching between thermal distribution 1 and 2. (default value 0.5)	0.5
src_2012	bool	Flag to apply 2012 MCNPX-derived, wavelenght-dependent correction to intensity from the cold moderator.	1
tfocus_dist	m	Position of time window	0.1
tfocus_time	s	Time position of window	0.0
tfocus_width	s	Time width of window	0.0
beamport_angle	deg	Direction within the beamport sector (0 < angle < 60) to direct neutrons	30

AT (	,	,		) RELATIVE
ROTATED (	,	,		) RELATIVE

The ESS_moderator_long Component

Identification

Description

Input parameters

Links

The `ESS_moderator_long` Component